1. Field of the Invention
The present invention relates to a semiconductor device having a thin-film element formed on a semiconductor substrate via an insulating film and, more particularly, to a semiconductor device having a thin-film inductor element as the thin-film element.
2. Description of the Related Art
Recently, a semiconductor device having a so-called CSP (Chip Size Package) structure in which the sizes of a semiconductor chip and package are almost equal has been known, and has been used to increase the packaging density of a circuit board. A semiconductor device having this CSP structure has a plurality of posts for connecting to, e.g., an external circuit connected to connection pads of a semiconductor chip, and is connected to a circuit board by forming a solder ball on the upper surface of each post.
To form a semiconductor chip having, e.g., a radio-frequency wireless communicating function, it is necessary to form, in the semiconductor chip, various passive elements such as capacitor elements and inductor elements for implementing the RF function of a PLL circuit, VCO circuit, or filter circuit. Since these passive elements require a relatively large area, the chip area inevitably increases if these passive elements are incorporated into the semiconductor chip. Therefore, it is sometimes necessary to form these passive elements by forming thin-film elements on an insulting film on a semiconductor substrate having an integrated circuit, and connecting the passive elements to the integrated circuit, thereby suppressing the increase in chip area and increasing the packaging density.
When forming a thin-film inductor element as a thin-film element, the thin-film inductor element is formed by forming spiral conductive layers between the two terminals. If the two terminals of this thin-film inductor element are formed on the same plane and the number of turns of the spiral conductive layers is made larger than one in order to increase the inductance value of the thin-film inductor element, the conductive layers forming the thin-film inductor element necessarily intersect each other in a certain portion.
In this intersecting portion of the conductive layers, the two conductive layers intersect each other via the insulating film on the semiconductor substrate; one conductive layer is a lower conductive layer formed below the insulating film, and the other conductive layer is an upper conductive layer formed above the insulating film.
The lower conductive layer is closer to the substrate than the upper conductive layer. When a semiconductor substrate is used as the substrate, the inductor characteristic value (Q value) decreases under the influence of the substrate in the lower conductive layer close to the substrate. That is, when an electric current flows through the lower conductive layer, an eddy current also flows through the semiconductor substrate by induction because the electrical resistance of the substrate is relatively low. A loss produced by this eddy current causes a phenomenon in which the Q value of the thin-film inductor element decreases. The closer the lower conductive layer to the terminal of the inductor element, and the larger the length of the lower conductive layer, the larger the influence of the decrease in Q value.
Also, if the intersecting portion of the conductive layers is close to one of the two terminals of the thin-film inductor element, the Q value of the inductor viewed from this terminal becomes smaller than that viewed from the other terminal, and this makes the inductor characteristics viewed from the two terminals different from each other.
The inductor element is required to have high performance with a high Q value, and the inductor element is required to have characteristics which are symmetrical when viewed from the two terminals. That is, the inductor element is a two-terminal element and has no polarity. When using the inductor element as a circuit element, therefore, it is generally unnecessary to take account of a terminal to be connected to the circuit, so the characteristics viewed from the two terminals must be equal. However, the inductor characteristic values viewed from the two terminals are different in the thin-film inductor element having the arrangement as described above, and this makes the element very difficult to use as a circuit element.